One semiconductor process in which substrate temperature is critical is thin film deposition in vacuum. The substrate temperature has a large effect on the thin film deposition process and the quality of the resulting films. One known thin film semiconductor device is a CdS/CdTe photovoltaic (PV) module. This device has thin films of CdS and CdTe semiconductors on a soda lime glass substrate. A known configuration for a CdS/CdTe PV module is the back wall or superstrate configuration. In this configuration, the soda lime superstrate (referred to in all other cases described hereinafter as the substrate) is most commonly coated with a transparent conductive oxide (TCO) film onto which the other films are deposited in the following order: a) a CdS film; b) a CdTe film; c) an ohmic contact layer; and d) a back electrode layer. Along with these film depositions, several heat treatment steps are also required to enhance device properties. It has been found that in order to produce the optimum CdS/CdTe PV device using a process such as that described in U.S. Pat. No. 6,423,565 it is desirable to cool the soda lime glass substrate and the adherent TCO/CdS/CdTe films to about 25° C. before applying the ohmic contact layer. The soda lime glass substrate reaches temperatures on the order of 200° C. in the previous processing step. It would be desirable to provide an apparatus for cooling the soda lime glass substrate from about 200° C. to 25° C. in a time period of 2-4 minutes with the substrate maintained in the vacuum chamber at a process pressure of about 40 mTorr. In the past, a time period on the order of hours was required to cool the substrate to the optimal temperature by relying on radiation cooling alone.
In order for this prior art CdS/CdTe PV module to serve as a practical low cost energy conversion device for converting sunlight into electricity, the soda lime glass substrate must be of a large size. Known CdS/CdTe PV modules are fabricated in a nominal 2 ft.×4 ft. size. The CdS/CdTe PV module must also be capable of being manufactured at the lowest possible cost and at a high volume throughput.
There are three possible physical mechanisms for cooling an object: 1) convective cooling resulting from movement of the surrounding gas (this gas movement can be natural due to density changes in the gas or may be forced, for example, by a fan); 2) conductive cooling resulting from direct contact of the object with a colder surface (conduction can be assisted by a contact medium including a thin layer of gas); and 3) electromagnetic radiation of thermal energy to colder surroundings. In the case of a hot object with no cold surfaces in close contact, only radiation cooling is effective at vacuum pressures. The rate of radiation cooling is governed by the difference in temperature between the hot object and the colder surroundings. The thermal radiation cooling rate is proportional to the quantity Thot4−Tcold4. Thus, the cooling rate is quite slow in the practical case of a semiconductor substrate below 200° C. surrounded by colder surfaces at 25° C. at some finite distance away and with an intervening ambient at vacuum pressures of 40 mTorr.
There are known methods in the prior art for cooling substrates in vacuum by using gas phase conductive cooling in combination with a cold surface in close proximity. U.S. Pat. No. 6,907,924 to Moslehi, for example, teaches the use of a thin heat transfer region between the substrate and the substrate holder or wafer chuck. Gas flowing through these thin heat transfer regions is the transfer medium employed to either heat or cool the substrate in a vacuum. The planar surface of the substrate holder may be heated or cooled, as desired. The prior art teaches heat transfer through a thin heat transfer region on only one side of the substrate. This teaching is disadvantageous when attempting to cool large area substrates, such as those employed in the apparatus and process of the present invention. In order to cool a 4-mm thick soda lime glass substrate from 200 C to 25 C in a period of 2 minutes, it has been found that the pressure in the intervening thin heat transfer region must be on the order of 10 Torr. In order to bring the gas pressure to this level it is necessary to seal the edges of the thin heat transfer region. It has also been found that the cooling rate of the substrate is a function of the spacing between the substrate and the cooled plate. For spacing distances larger than 0.005 in., the cooling rate rapidly drops to zero. If 10 Torr of pressure is applied to only one side of a large area soda lime glass substrate and the ambient processing pressure on the opposite side of the substrate is 40 mTorr or essentially zero, a considerable amount of deflection of the substrate will occur. This deflection also introduces stress and may lead to breakage of the substrate. Using equations from a standard reference such as the “Machinery's Handbook”, 24th addition, published by Industrial Press Inc. New York, ISBN 0-8311-2492-X, it is possible to estimate the deflections and stresses in flat plates subjected to an evenly distributed load such as gas pressure. Given a 2 ft.×4 ft. panel of soda lime glass clamped top and bottom on all edges and a pressure of 10 Torr applied to one side only, the maximum deflection will be approximately 0.220 in. at the center of the panel. This degree of deflection will drive the heat transfer rate to zero over much of the panel area. In addition, the introduced stress for the same case will be approximately 2900 psi, which exceeds the 0.8% breakage design.
Thus, the prior art substrate cooling methods are ineffective to provide high throughput, low cost cooling of large area substrates in vacuum without the risk of damaging the substrates or their adherent films.